Influence of coal structure change caused by different pretreatment methods on Shengli lignite pyrolysis

Coal pyrolysis is one of the methods of clean and efficient utilization of coal. Coal has a complex structure, which has an important influence on its pyrolysis characteristics. In order to further explore the structure–activity relationship between the structure and pyrolysis characteristics of coal, the structure of coal can be artificially changed to investigate the influence of structural changes on its pyrolysis characteristics. In this study, hydrothermal pretreatment (HTP) and solvent swelling pretreatment (SSP) were used to artificially change the structure of Shengli (SL) lignite. The influence of each pretreatment on the physicochemical structure was analysed by Fourier-transform infrared (FT-IR), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The evolution of functional groups and the pyrolysis characteristics of the SL/pretreated coal were investigated by using an in-situ FT-IR device and a fluidised bed reactor, respectively. The results showed: HTP could remove some oxygen-containing functional groups and aliphatic side chains, hydrolyze ether bonds and generate phenolic hydroxyl groups, thus destroying the cross-linking structure, promote the release of aromatic CC, the yield of pyrolysis tar increased from 9.4% to 10.6%, and the content of light tar increases from 43.1% to 49.1%. SSP mainly weakening the cross-linking structure by destroying hydrogen bonds and dissolving part of oxygen-containing functional groups, which reduces the pyrolysis activation energy of oxygen-containing functional groups and aliphatic groups, promotes the generation of tar. Accordingly, the yield of tar increases from 9.4% to 11.1%, and the content of light tar increases from 43.1 % to 49.8%. In particular, this research work reveals the structure–activity relationship between the structural changes of coal and the pyrolysis characteristics, which provides a theoretical basis for the directional pyrolysis of lignite.